1. Field of the Invention
The present invention relates to a binding device for forming a fold line on a booklet and an image forming apparatus having this.
2. Description of the Related Art
Conventionally, a binding device for folding a sheet outputted from a main body of an image forming apparatus such as a copying machine and a printer in half and sewing the inside of a book has been known.
The constitution of such a binding device is disclosed in Japanese Patent Application Laid-Open No. 61-2671. In addition, FIG. 16 shows a schematic block diagram of a main body of a conventional image forming apparatus 10 and a binding device 11 on their cross sections, which are disclosed in JP-A No. 61-2671.
As shown in FIG. 16, the binding device 11 is provided with a stack tray 12 as a sheet stacking portion, a folding knife 15 as a folding portion, and a folding roller pair 16 as a fold line forming portion.
On the stack tray 12, an opening portion 14 is disposed on its center portion, and a stopper 13 for stopping the sheet on a fixed position is disposed on an end portion of the stack tray 12. In addition, on the upper part of the opening portion 14 of the stack tray 12, the folding knife 15 is disposed so as to be capable of rising and falling.
When the sheets are conveyed from the main body of the image forming apparatus 10 to the binding device 11, the conveyed sheets material are stacked on the stack tray 12 in series.
Then, the folding knife 15 that is disposed upward of the stack tray 12 falls on the sheets stacked on the stack tray 12, and the folding knife 15 strikes the center portion of the sheet so as to fold the sheet and push the sheet into the opening portion 14 of the stack tray 12. In the following explanations, a plurality of sheets stacked on the stack tray 12 being folded down calls a booklet.
Sheets pushed into the opening portion 14 of the stack tray 12 is folded down to be made into a booklet. After that, the booklet is provided to the folding roller pairs 16 being folded down. The folding roller pairs 16 forms a fold line on the center portion of the booklet by conveying the booklet provided while sandwiching it from the opposite sides. The booklet having the fold line formed thereon is conveyed on a tray 17 due to rotation of the folding roller pairs 16 to be stacked on the tray 17.
However, when a pressure applied by the folding roller pair 16 is released, the booklet is opened by its own elasticity and a fold line of the booklet formed by the conventional bookbinding apparatus 11 is occasionally returned to the original state. As a result, the booklet has a poor-looking appearance such that the fold line is opened.
The problem relating to the opening of the fold line is most obvious in the case where sheets whose opening amount is large are used as sheets composing a booklet. The sheets whose opening amount is large mean sheets which are once folded but are easily opened including sheets such as cardboard whose rigidity is high and sheets such as thin paper whose rigidity is low but whose periphery has a small water amount in a low-humidity environment.
When a fold line is formed by bending sheets whose opening amount is originally small to a direction orthogonal to a fiber orientation of the sheets, the sheets are easily opened greatly. Such sheets are, therefore, regarded as sheets whose opening amount is large.
The problem relating to the opening of the sheet is not eliminated to not less than a certain degree even by increasing a pressure to be applied by the folding roller pair 16. For this reason, solving means other than the method for increasing the pressure by the folding roller pair 16 is required. As one example of the solving means, Japanese Patent Application Laid-Open No. 11-157247 discloses a method for humidifying sheets using a humidifying unit so as to ease the forming of fold lines of the sheets.
FIG. 17 is a schematic block diagram on a cross section of the binding device 11 having humidifying unit disclosed in JP-A No. 11-157247.
The bookbinding device 11 shown in FIG. 17 is provided with a humidifying chamber 18 as a humidifying unit, humidifying nozzles 19 and 20, and a heating roller pair 30 as a heating portion.
In FIG. 17, the humidifying chamber 18 humidifies sheets which are being conveyed on an upper stream side of the stack tray 12. The humidifying nozzle 19 humidifies a part of a booklet to be folded on an upper stream side of the folding roller pair 16. The humidifying nozzle 20 humidifies a part of a booklet to be folded on a lower stream side of the folding roller pair 16.
When these humidifying units 18, 19, and 20 humidify a part of a booklet to be folded, even if sheets whose opening amount is large are used, before a fold line is formed by the folding roller pair 16, the opening amount of the sheets can be reduced. Therefore, a fold line can be easily formed by the folding roller pair 16, so that a good-looking booklet without opening can be provided.
In the bookbinding apparatus 11 having the humidifying units disclosed in Japanese Patent Application Laid-Open No. 11-157247, however, a humidifying condition is not changed according to sheets in which a fold line is formed. For this reason, in the case of a booklet made of particularly sheets which water hardly penetrates such as sheets whose surface is specially treated, the sheets are curled to a direction where the booklet opens, and thus the booklet might have a poor-looking appearance.
In the conventional humidifying units, since the humidifying chamber 18 is constituted so as to humidify also a portion of a sheet which is not folded, this unnecessary humidifying causes inefficiency of the device.
As shown in FIG. 17, since the humidifying nozzles 19 and 20 humidify only a sheet on an outmost position of a booklet in the sheets composing the booklet, it is difficult to humidify sheets on an inner position of the booklet.
As a result, the opening amount of the sheets on the inner position with respect to the sheet on the outer position of the booklet cannot be sufficiently reduced, and thus the booklet might have a poor-looking appearance where the opening occurs.
In addition, the portion of the sheet that is humidified by the humidifying nozzle 19 is exposed from an opening portion 14 of the stack tray 12 to the outside. Therefore, water given by the humidifying nozzle 19 evaporates around the area till the booklet is folded down on the inside by the folding roller pairs 16.
As a result, since the opening amount of the sheet cannot be sufficiently reduced, the booklet might have a poor-looking appearance where opening occurs.
A phenomenon from the humidifying of the sheets to the occurrence of a curl is described with reference to FIGS. 18A, 18B, and 18C.
FIGS. 18A, 18B, and 18C are pattern diagrams showing a cross section of humidified sheets. In the drawings, a portion humidified by the humidifying units (hatched portion in the drawings) once stretches just after humidifying, but when the sheets are again dried, forces to a direction where the sheets further shrink with respect to the shape before humidifying are generated as shown by arrows in the drawings.
On the other hand, a portion which is not humidified is hardly influenced by humidifying and drying, and thus it does not stretch nor shrink. Therefore, a shrinkage amount is different between the portion to be humidified and the portion which is not humidified. As a result, when the sheets are humidified, the sheets are easily curled to a direction where the humidified surfaces of the sheets shrink.
That is, as shown in FIG. 18C, in the case where the inner side of the booklet shrinks due to the humidifying of the inner side, the sheets are curled to a direction where the booklet is closed. As a result, the sheets do not open, and the booklet does not have a poor-looking appearance.
As shown in FIG. 18A, however, since the humidifying nozzles 19 and 20 humidify the outer side of the booklet, the outer side of the booklet shrinks. Therefore, the sheets are curled to the direction where the booklet is opened, the opening occurs, and the booklet might have a poor-looking appearance.